Show where an "incompatible type" was first declared

#2380 shows the need to find out where an "incompatible type" error was first declared. In big code bases, it will otherwise be near impossible to find which superclass defined a variable, causing an error down the chain.

Create a PR for it in my own fork, as the code depends on a pending PR.

Superseded by other tickets; will open PRs for those I think I can fix.